Designing For Tolerances

[glencandle]

Hi, I am a CNC enthusiast, teaching myself how to design parts and have them manufactured, which is lots of fun and I hope to someday transition careers using this skillset.  That said, there is one thing I still don't have a definitive answer on so I wanted to see if any pros could weigh in:

When I'm designing two parts that are meant to fit together (i.e. lid and body), how do I account for tolerances? 

For example, if the shop guarantees a tolerance of +/-0.005" then do I add 0.005" to one part and subtract that amount from the other (where the dimensions fit, for example, a lid and body)?  In other words, the lid of a cylinder would be 

Or should I design both parts to have the same outer/inner dimensions where they fit, and then note the tolerance in a drawing (e.g. the body tolerance would be 0" to -0.005" and the lid tolerance would be 0" to +0.005")?

Cheers.

[seth.madore]

Well, technically, they could machine both items and hug one side of the tolerance. The parts would meet print, but not able to be assembled (without some...persuasion).

Thus, assuming items are not meant to be an interference fit, we always want to have some measure of clearance even when the MFG'er rides a tolerance at the high/low.

[glencandle]

Hey Seth, thanks for the words of wisdom.  This makes perfect sense. Not meant for interference, but definitely want a snug fit.  Is there a rule of thumb for the amount of 'extra' you'd want to factor in, per inch (or factor), for fitted parts? 

I just prototyped two parts (lid and body) that are about an inch where they fit, and added +0.005 to the inner width of the lid, and the -0.005 to the outer of the body, and the parts seem to fit perfectly.  Not sure if I just got lucky per the MFG tolerance or if I'm right on the money for this type of design.

Cheers.

[leo.castellon]

Engineers take a whole class on this subject. Regarding machine shops and tolerances, remember that the tighter the tolerance, the higher the cost of the part. Here is a link to an article that just touches on the subject of tolerance stacking: Best Practices of Tolerance Stacking - In The Loupe (harveyperformance.com) .

LeoC

[glencandle]

Hugely helpful article, thank you!!